3AJ swing arm/rear suspension query
 

recently i noticed a problem with the swing arm on my 3AJ - the chain was resting on the top of the swing arm and there was a lot of slack in the chain. i thought that if i increased the tension on the chain the chain would be lifted clear of the swing arm, but it is actually the positioning of the swing arm in relation to the sprocket that seems to be causing the problem.

i suspected that the rear shock had collapsed and removed it, but upon closer inspection the mono shock seemed to be fine.

someone suggested to me that it might be the swing arm linkage bearings which might have seized up, thus preventing the mono-shock from expanding to its intended size. does this sound reasonable?

has anyone experienced this problem before? any advice would be much appreciated.

Ok, let's do some 2,5 dimensional thinking.

Looking from the LH side we have, from front to back, the drive shaft from the gearbox, the pivot shaft of the swing arm and the rear axle.
1. When the bike is not loaded the center of the rear axle should be below the line drawn through drive shaft and pivot shaft. The length of chain you need in this situation is shorter than in the next:
2. You get on the bike, via the lever system the shock/spring aasy is loaded and the rear axle moves up with respect to aforementioned line. The length of chain you need now is longer than in situation 1!

Remark: you need the longest length of chain when drive shaft - pivot shaft- rear axle are in line, which should only happen while bumping over something!

This means, that when the bike is not loaded, with a correctly tensioned chain, there seems to be a lot of slack, which might cause the chain to rest on the plastic thingy mounted on top of the swing ar, close to the pivot. You will see wear marks in the plastic corresponding to the chain.

For the same reason, a chain that is correctly tensioned on the unloaded bike is too tight, causing wear on chain, sprockets, and worst of all the outer drive shaft bearing. In the extreme it might impede proper suspension action as the chain effectively takes over the function of the shock.

So, with chain adjustment, err on the loose side. I do it as follows: sitting on the bike it should still be possible to easily move up the chain to touch the under side of the swing arm where the plastic thing is.
Also, check the two plastic rolls bolted to the frame close to the pivot. They should be there and their purpose is to take up some of the slack required as explained above.

Auke

hi auke

thanks for the reply. the chain actually rest on the swing arm when i am on the bike riding, not only when there is no weight on the bike.

thanks
cabron

Strange. Compared to other Xt's, how does the suspension feel? Very hard and not supple? If so, indeed something might have seized but that would be very extreme. And your spine would have given up by now.

Other possibility, did you change the spring or did you set it for carrying loads of luggage?

Auke